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Old 25th Jan 2018, 11:39 am   #1
M0FYA Andy
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Default Wire-wound resistor question

Thinking about the large wire-wound tapped or variable resistors, wound with nichrome or similar wire on a ceramic tube, for example Radiospares droppers or physics laboratory resistors, what provides the insulation between adjacent turns?
Is it a layer of oxide on the metal, and does this just exist naturally?
Looking carefully at examples, the turns look to be in contact, so not wound on a spiral cut in the ceramic.
I'm struggling to find one which meets my requirements (6" long, 1" or a bit bigger diameter, resistance 8 ohms), so thinking about winding my own on the tube recovered from an existing one of the wrong resistance.

Many thanks,

Andy
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Old 25th Jan 2018, 11:41 am   #2
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Default Re: Wire-wound resistor question

I had to rewind the dropper resistors of my Ekco AD65 and the wire certainly had an insulating coating of some kind, probably an oxide.
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Old 25th Jan 2018, 12:38 pm   #3
David G4EBT
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Default Re: Wire-wound resistor question

Quote:
Originally Posted by M0FYA Andy View Post
I'm struggling to find one which meets my requirements (6" long, 1" or a bit bigger diameter, resistance 8 ohms), so thinking about winding my own on the tube recovered from an existing one of the wrong resistance.

Andy

You don't mention what it's for Andy, but to get a tubular wirewound resistor of the dimensions you state means it would be 100 Watts. This for example from Ali Express, who sell either fixed ones, or ones with slider adjustment. This example is 8 Ohms, 100 Watts, and is 160mm long, (just over 6 inches) but they're available in other lengths and diameters too. I've bought similar higher resistance ones with several sliders for use as mains droppers on AC/DC sets where space permits:

https://www.aliexpress.com/item/8-oh...ceBeautifyAB=3

Some are available non-inductively wound, for use as dummy loads, such as this one, but they're longer - this being almost 9 inches:

https://www.aliexpress.com/item/8-oh...ceBeautifyAB=3

You can of course get smaller 100 Watt metalclad wirewound resistors, but the rating is when mounted on an adequate heatsink, so you'd need to de-rate it at least 50%, but in any event, the style is inconsistent with your dimensional spec:

https://www.ebay.co.uk/itm/100W-Watt...-/282755841093

Hope that helps a bit.
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Old 25th Jan 2018, 12:58 pm   #4
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Default Re: Wire-wound resistor question

FWIW it might be easier to source 8.2 ohms.......
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Old 25th Jan 2018, 2:08 pm   #5
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Default Re: Wire-wound resistor question

Asked this insulation question long ago. The wire is oxidized, and burnt in after winding.
We used resistance wire as a heater for cutting glass tube, one complete loop around the tube, the hot spot then cooled with saliva cracks it round.
Where the wire crossed, it did not short together.
This used to be the problem when fitting connecting bands around droppers, getting a good contact was hard but once tightened the wire made contact and did not corrode again.
Can you not fit a tap on an existing dropper?
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Old 25th Jan 2018, 5:01 pm   #6
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Default Re: Wire-wound resistor question

Perhaps I should have made my application clearer, but thought it better to stick to the generic question of how is the wire insulated rather than muddle it with a request for a suitable resistor!

It's all part of my project to recreate an Oscillator Type 217, which is a controller for the 24 volt scanner motor in a WW2-era ASV MkXI radar. I posted a question a couple of weeks ago to ask if anyone had such a unit surplus to requirements (no reply, but it was a 'long-shot'), and my recent question about PO600 relays is also relevant.
I've managed to borrow an incomplete unit from a pal of mine, to photograph and measure. The resistor in question is present (but the relays aren't), so I'll get a photo showing that as a priority.
Although 8 ohms, it is isn't used end to end, so 10 or 12 ohms would be OK. It does however have two adjustable tappings, so the types mentioned by David wouldn't do.
I have ordered a 10 ohm 100 watt resistor from Hong Kong, ebay item 121765304695, but it hasn't arrived yet, so I'm not sure whether it will fit, with its feet removed. Incidentally it looks like this one avoids the insulation question by having a space between each turn.

Maybe if I do have to resort to winding a resistor I could use wire recovered from an example I have which is physically too big but must have the layer of oxide present?

Thanks for the interest,

Andy
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Old 26th Jan 2018, 1:58 am   #7
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Default Re: Wire-wound resistor question

Years ago, I had to re-wind some large variable resistances.
Brand new and shiny nichrome wire was indeed vulnerable to short circuits between turns.
To prevent this it was desirable to heat the wire before use in order that the insulating oxide film would form. Briefly running it at a bright red heat did this.

This is easily done by passing current through the wire whilst stretched out in the open, or LOOSELY wound on an insulating and fire proof former.
Depending on the resistance, a vehicle battery or the mains could be used, taking care of course.

Nichrome wire from old heating elements or heavily loaded resistors becomes very brittle and often can not be re used.
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Old 26th Jan 2018, 2:03 am   #8
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Default Re: Wire-wound resistor question

Quote:
Originally Posted by Herald1360 View Post
FWIW it might be easier to source 8.2 ohms.......
Not in my experience with high power resistors.
Small resistors are of course usually in the preferred values, but high wattage ones tend to be in round figures rather than preferred values, 50 ohms rather than 47 ohms, or 8 ohms rather than 8.2 for example.
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Old 26th Jan 2018, 8:43 am   #9
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Default Re: Wire-wound resistor question

Hi Andy, I have a selection of resistance wire of various sizes and values including oxidised types.
Let me know if you need any

Ed
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Old 26th Jan 2018, 10:23 am   #10
M0FYA Andy
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Default Re: Wire-wound resistor question

Ed, many thanks for your offer!
I'll get a picture today of the resistor in the unit I've borrowed so everyone knows what I'm looking for or must make. I'll wait until the one from HK arrives before deciding the way forward, unless someone says 'I've got exactly what you need'.
Andy
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Old 26th Jan 2018, 3:32 pm   #11
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Default Re: Wire-wound resistor question

This is the resistor in the unit I borrowed. Sorry it's not a very good picture!
Andy
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Old 27th Jan 2018, 9:23 am   #12
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Default Re: Wire-wound resistor question

Hi Andy, from memory my oxy-ferry wire is about 20ohms/ mtr, but note that it can be bifilar wound to reduce its value.

Ed
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Old 10th Feb 2018, 1:04 pm   #13
M0FYA Andy
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Default Re: Wire-wound resistor question

The resistor from HK arrived. It's 7.25 inches long, so won't fit in the 6 inch space available. However, I think I might take a pragmatic approach and build a 'pseudo-replica' 1.25 or 1.5 inches longer than the original. As well as giving space for this longer resistor, it would also allow me to fit in three PO3000 relays, in place of the PO600 types which must have been used originally (as discussed on another thread). PO3000 types are much more readily available, indeed I have a good stock of them.
Andy
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